/*
 * C.cpp
 *
 *  Created on: Jun 14, 2013
 *      Author: SOFTFOUNDRY-VN
 */
#include <iostream>
#include <string>
using namespace std;

long long x, y, m, tmp, depth;
int main() {
	depth = 0;
	cin >> x >> y >> m;
	if (x < y) {
		tmp = x;
		x = y;
		y = tmp;
	}
	if (m > x) {
		if (x <= 0) {
			cout << "-1\n";
		}
		else {
			if (y < 0) {
				tmp = -y;
				tmp = tmp / x;
				y = y + x * (tmp+1);
				depth = (tmp+1);
			}
			int d = 0;
			do {
				tmp = y;
				y = x;
				x = x + tmp;
				d++;
			} while(x < m);
			cout << depth + (long long) d<< "\n";
			//1100087778366101931
		}
	}
	else {
		cout << "0\n";
	}
}
